In addition to shielding the interior of a building from wind and rain, the roof of the building can reflect sunlight that impinges on the building. Accordingly, particularly in warm climates, building roofs are often made of light (e.g., white) materials to increase the reflectivity of the roof and aid in keeping the interior of the building cool. One drawback with such roofs is that they accumulate dirt over the course of time, which reduces the reflectivity of the roof and therefore the ability of the roof to keep the building interior cool. One approach to addressing this drawback is to periodically clean the roof, for example, by pressure washing or scrubbing the roof. However, this process is labor-intensive and typically uses a significant quantity of water, which is not always readily available in the warm climates where such roofs are most useful. In addition, typical roof cleaning processes include using detergents and surfactants, which are then washed down the building gutters into storm sewers and/or other channels that in turn direct the contaminated water into streams, lakes, aquifers and/or other natural environmental areas without treating it. Still further, non-reflective and reflective roofs can also suffer physical damage from debris buildup. For example, as organic materials build up on the roof's surface, they support the growth of fungi and/or moss, which can damage the roof structure. Accordingly, there remains a need for improved systems and techniques for cleaning roofs.